Kenichi Takita

Ignition and flame-holding by oxygen, nitrogen and argon plasma torches in supersonic airflow

Abstract The behavior of ignition and flame-holding of various plasma torches in supersonic flow were numerically investigated. To understand the role of radicals involved in the plasma jet (PJ), three kinds of PJs (O2, N2, and Ar) were simulated. For fuel injection both upstream and downstream of the PJ, the three PJs showed no difference with respect to the location and the behavior of ignition. Ignition occurred at the upper part of the rear jet, where the front jet turned upward because of passing through the interaction shock wave collided with the rear jet first. Differences appeared in the behavior of flame-holding after ignition in the case of upstream fuel injection. The flames ignited by the N2 PJ and Ar PJ blew out, but the one ignited by the O2 PJ was held by the O2 PJ itself. This result suggests that the local equivalence ratio is an important factor for flame-holding by the PJ. Moreover, flame propagation in the merged low Mach number region between the PJ and the H2 jet was observed in cases of the N2 PJ and the O2 PJ. On the other hand, there was no significant difference in three PJs in the behavior of the flame in the case of downstream fuel injection. In this case, reactions occurred mainly along the thin mixing layer of the PJ and the H2 jet. The performance of the Ar PJ was worse than those of the O2 PJ and the N2 PJ for all simulations. Therefore, radicals included in the PJ enhanced the combustion reaction and enlarged the combustion region.

Ignition Characteristics of Methane and Hydrogen Using a Plasma Torch in Supersonic Flow

Ignition and flame-holding characteristics of methane and hydrogen using a plasma torch igniter were experimentally investigated in a supersonic airflow. The main airflow Mach number was 2.3, and the stagnation pressure and temperature corresponded to atmospheric conditions. Nitrogen, oxygen, a hydrogen/nitrogen mixture, and a methane/nitrogen mixture were used as feedstocks for the torch. The fuel was vertically injected from the wall where the plasma torch was attached. The wall pressure and the total temperature at the exit of the test section were measured. Ignition was confirmed for hydrogen injected both upstream and downstream of the torch. No strong dependence on the kind of feedstock of the torch for effectiveness of ignition of the hydrogen fuel was evident in a supersonic flow. By contrast, ignition of the methane fuel was confirmed only when it was injected upstream of the torch. In addition, the wall-pressure increase of the methane fuel was about half that of the hydrogen fuel. An important result for methane fuel was that using oxygen as a feedstock resulted in the most remarkable increase of the total temperature and the wall pressure.

Ignition Characteristics of Plasma Torch for Hydrogen Jet in an Airstream

The ignition characteristics of a plasma torch for a hydrogen jet injected parallel to a subsonic aire ow were experimentally studied. Theregion of the injector position whereignition occurred became gradually narrow with an increase in the distance between the fuel injector and the plasma torch and steeply narrow with an increase in the aire ow velocity. This suggests that the ignition limit strongly depends on the penetration height of the plasma torch, which is in inverse proportion to aire ow velocity. Nitrogen and oxygen were compared as feed stocks. Results obtained showed no difference in the behavior of the plasma jet itself, the ignition limit, and the e ame shape. Calculations of ignition delay time for an H 2/air mixture with the addition of N and O radicals showed the sameeffectivenessand werefoundto besuperiorto theHandtheOHradicals.However,thedegreeofdeterioration of an anode nozzle made of copper was more severe for oxygen plasma than for nitrogen.

Combined effects of radiation, flame curvature, and stretch on the extinction and bifurcations of cylindrical CH4/Air premixed flame

Abstract The combined effects of flame radiation, stretch, and curvature on the extinction and flame bifurcations of the cylindrical premixed CH 4 /air flames are numerically investigated with a detailed chemistry. The interaction between radiation heat loss and flame curvature is emphasized. The results show that a mixture below the standard limit can burn in the cylindrical flame configuration by imposing a moderate stretch rate. This flame is quenched either by radiation heat loss at low stretch rate or by incomplete combustion with an excessive stretch. As the fuel concentration increases, it is found that two kinds of flames, normal flame and weak flame, can exist at the same boundary conditions. A G-shaped extinction curve showing the flammable regions of these two flame regimes is obtained. The relation between the flammability limit of the cylindrical flame and the standard limit is discussed. Furthermore, comparisons between the cylindrical flame and the counterflow flame are made. The results show that the interaction of flame curvature and radiation heat loss greatly affects the flame strength and extinction. It is shown that flame curvature extends the radiation extinction limit and accelerates the opening up of the sublimit flame branch. Comparisons between the predicted results with the experimental data show good agreements both in the extinction limit and in the extinction flame diameter.

Ignition and flame-holding of H2 and CH4 in high temperature airflow by a plasma torch

Abstract The effect of airflow temperature on ignition characteristics of a plasma torch was experimentally investigated. Various combinations of fuels (CH 4 and H 2 ) and plasma jets (PJs) (N 2 , O 2 , and N 2 /H 2 ) were tested for a wide range of airflow temperature from 300K to 700K and the PJ power required for ignition was investigated. Ignition by the PJ occurred more easily in a high temperature airflow than in an atmospheric temperature airflow. The main reason for this was considered to be the increase in the reactivity of the fuel at high temperature rather than the effect of radicals in the PJ, because there was no difference in the spectroscopic measurement of the PJ between T air = 300K and 700K. The addition of H 2 to the N 2 feedstock was also effective for ignition enhancement of both fuels (H 2 and CH 4 ). In particular, the H 2 30%/N 2 70% PJ was able to ignite both fuels even at atmospheric temperature and the lowest electric power input for the stable operation. One of the reasons for this advantage of the H 2 30%/N 2 70% PJ was the heat release from the diffusion flame of the H 2 included in the feedstock with the airflow after injection. Moreover, the conditions around the local ignition site such as the local fuel concentration or the size of the contact area of the PJ and the fuel jet were found to be important factors for the success of ignition.

Correction Method for Particle Velocimetry Data Based on the Stokes Drag Law

A correction method based on the Stokes drag law was developed for flow velocimetry using tracer particles. The present method can be used to calculate the corrected velocity from the measured velocity and its spatial gradient without the need to solve the whole flowfield. Velocity measurement was conducted for sonic transverse injection into Mach 1.8 flow using particle image velocimetry. The correction method was applied to the velocity field obtained from the particle image velocimetry data. The corrected data showed the positions and shapes of the shock waves in the measurement region more clearly. The corrected peak velocity downstream of the shock waves became closer to the theoretical values. The jet streamlines calculated from the corrected and original particle image velocimetry data were compared. The streamlines from the corrected data went lower than those from the original data.

Effects of Airstream Mach Number on H/N Plasma Igniter

Experimental study of the plasma igniter was conducted in nonheated high-speed air stream. The effects of the Mach number, from 0.9 to 2.4, and the stagnation pressure of the airstream, from 64 to 100 kPa, as well as the hydrogen fraction in the H 2 /N 2 feedstock of the torch, from 0 to 50%, were investigated. In the higher Mach number airstream, more input power to the torch was required to ignite hydrogen fuel. The stagnation pressure of the airstream had complicated effects relating to chemistry and aerodynamics. Improvement of ignition performance due to the increased hydrogen fraction in the feedstock was well correlated with the sum of the electric input energy and the heat of combustion of the hydrogen fed as the feedstock of the torch.

Large-Eddy Simulation of Jet in Supersonic Crossflow with Different Injectant Species

Nomenclature C = molar concentration, mol=m cp;k = specific heat at constant pressure for species k, J= kg K D = injector diameter, m Dk = diffusion coefficient for species k, m =s E = total energy per unit mass, J=kg f = frequency, 1=s H = total enthalpy per unit mass, J=kg h = enthalpy per unit mass, J=kg hk = enthalpy per unit mass for species k, J=kg H j = subgrid-scale total enthalpy flux vector, J= m s h = subgrid-scale species mass fraction-enthalpy correlation, J=kg J = jet-to-crossflow momentum flux ratio k = turbulent kinetic energy, m=s k = subgrid-scale kinetic energy, m=s M = Mach number m = mass flow rate, kg=s Mw = molecular weight, kg=mol p = static pressure, Pa Prt = turbulent Prandtl number pt = total pressure, Pa qj = heat flux vector, J= m s q j = subgrid-scale energy diffusion due to species diffusion, J= m s R = mixture’s gas constant, J= kg K Rk = gas constant for species k, J= kg K ReD = Reynolds number based on D rs = spatial correlation coefficient rts = time–space correlation coefficient Sij = rate of strain tensor, 1=s Sct = turbulent Schmidt number T = static temperature, K t = time, s Tt = total temperature, K T = subgrid-scale mixture gas constant-temperature correlation, J=kg T = reference temperature, K U = velocity magnitude, m=s u, v, w = velocity components in x, y, and z directions, m=s Uc = convection velocity, m=s ui = velocity component in xi direction, m=s Vj;k = species diffusion velocity vector for species k, m=s x, y, z = streamwise, transverse, and spanwise direction distances in Cartesian coordinates, m xi = Cartesian coordinates, m Yk = mass fraction for species k Y j;k = subgrid-scale species diffusion vector, kg= m s = specific heat ratio hf;k = standard heat of formation at T , J=kg in = mean boundary-layer thickness at inlet, m m = mixing efficiency sgs j;k = subgrid-scale species mass fraction-diffusion velocity correlation, kg= m s = mixture’s thermal conductivity, J= m K s = mixture’s molecular viscosity, kg= m s t = subgrid-scale eddy viscosity, m =s = density, kg=m ij = viscous stress tensor, Pa sgs j = subgrid-scale viscous work, J= m s sgs ij = subgrid-scale stress tensor, Pa ’ = local equivalence ratio

Effects of Combustion and Shock Impingement on Supersonic Film Cooling by Hydrogen

Supersonic film cooling with H 2 coolant was numerically investigated by considering the combustion of the coolant and the impingement of the shock wave. The influence of temperature fluctuation was included in calculating chemical rate constants. The combustion region was formed apart from the wall, and there was little heat release from the flame. Therefore, the combustion of H 2 had only a weak influence on the characteristics of the film cooling. The location of ignition moved upstream considerably due to the temperature fluctuations. However, the movement decreased the cooling efficiency very slightly. In addition, the effect of shock impingement on the film cooling by reactive H 2 was almost the same as that of nonreactive N 2 with the same coolant injection Mach number.

Characteristics of Hydrogen Jets in Supersonic Crossflow: Large-Eddy Simulation Study

The characteristics of hydrogen jets transversely injected into a supersonic crossflow under four different injection and crossflow conditions were investigated by large-eddy simulation. The effects of the jet-to-crossflow momentum flux ratio and crossflow velocity were studied. The jet trajectory in the averaged field was controlled by the value of the square root of the jet-to-crossflow momentum flux ratio irrespective of the crossflow conditions. When the crossflow conditions were fixed many jet characteristics were similar in the space normalized using the square root of jet-to-crossflow momentum flux ratio. On the other hand, the crossflow conditions had a strong impact on the jet characteristics. Although the turbulent intensity around the jet was not affected to a great extent, the shape and convection velocity of the large-scale structures appearing on the windward side of the jet plume depended on the crossflow conditions. With a higher crossflow velocity the convection velocity was higher, the j...